To increase the areal storage density of a magnetic recording device, the recording layer thereof may be provided with smaller and smaller individual magnetic grains. This reduction in grain size soon reaches a limit at which point the magnetic grains become thermally unstable and incapable of maintaining their magnetization. The thermal stability of the magnetic grains can be increased by increasing the magnetic anisotropy thereof (e.g., by utilizing materials with higher anisotropic constants). Increasing the magnetic anisotropy of the magnetic grains, however, increases their coercivity and therefore requires a stronger magnetic field to change the magnetic orientation of the grains (e.g., in a write operation).
Energy-assisted magnetic recording (EAMR) may be used to address this challenge. In an EAMR system, a small spot where data is to be written is locally heated to reduce the coercivity of the magnetic grains therein for the duration of the write operation, thereby allowing materials with increased magnetic anisotropy to be used, and greater areal storage density to be exploited.
In one EAMR approach, a semiconductor laser diode may be used as a light source and coupled to a planar waveguide which serves as light delivery path. A grating structure may be used to couple the laser light into the waveguide. The coupled light is then routed to a near field transducer (NFT) by which the optical energy is provided to a small optical spot on the recording media a few tens of nanometers (nm) in size.
NFT's are typically mounted on a waveguide to take advantage the surface plasmon waves to create local electro-magnetic fields. Light from the waveguide impinges on the NFT and causes a surface charge that can be transferred to an antenna portion of the NFT; and becomes useful for transferring energy from the antenna to a receptor such as recording media. NFT designs and antenna shapes vary; some known examples include disk type and aperture type. Some aperture type examples include “C” aperture, capital i or “I” aperture, hereinafter called an “H” aperture, triangle aperture, and bowtie aperture.